1. Field of the Invention
The present invention relates to a developing apparatus provided with a developer bearing member, and more particularly to a developing apparatus advantageously applicable to an image forming apparatus of an electrophotographic method or an electrostatic recording method such as a copying apparatus, a printer or the like.
2. Related Background Art
In an image forming apparatus such as a copying apparatus or a printer, an electrostatic latent image formed on an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member has been conventionally rendered visible with a powdered developer (toner).
As an example, there will be explained, with reference to FIG. 7, an image forming apparatus of reversal development type utilizing conventional non-magnetic one-component toner.
The image forming apparatus is constituted by an image bearing member (hereinafter referred to as photosensitive drum) for bearing an electrostatic latent image, rotated in a direction p, a charger 102, an exposure device 103 for forming an electrostatic latent image corresponding to image information on the photosensitive drum 101, a developing device (developing apparatus) 104, a transfer charger 105, a fixing device 106, a cleaner 107 etc.
The developing device 104 is provided with a developer bearing member 110 (hereinafter called developing roller), rotated in a direction q for conveying developer (hereinafter referred to as xe2x80x9ctonerxe2x80x9d) toward a contact portion with the photosensitive drum 101. The developing roller 110 is so-called elastic developing roller constituted by an elastic member formed on a conductive metal core, and is positioned in contact with the photosensitive drum 101. Around the developing roller 110, there are provided a removing-feeding roller 111 constituting a removing-feeding member rotated in a direction r, for feeding non-magnetic one-component toner onto the developing roller 110 and removing the toner from the developing roller 110, a regulating blade 112 constituting a regulating member for regulating the amount of the toner on the developing roller 110, and an agitating blade 113 constituting an agitating member for supplying the removing-feeding roller 111 with the toner. The removing-feeding roller 111 is constituted by a metal core shaft and a continuous cell foamed member formed thereon.
Referring to FIG. 7, the photosensitive drum 101 is rotated in a direction p, and the surface thereof is uniformly charged negatively by the charger (charging device) 102 to which a voltage is applied by an unrepresented bias source. Thereafter a laser beam from the exposure device 103 forms an electrostatic latent image on the photosensitive drum 101. Then, this electrostatic latent image is rendered visible as a toner image, by toner contained in the developing device 104 and conveyed by the developing roller 110 positioned in contact with the photosensitive drum 101. Thereafter, the toner image on the photosensitive drum 101 is transferred onto a transfer material 108 such as paper or an OHP sheet by a transfer charger 105, and is finally fused and fixed to the transfer material 108 by the fixing device 108.
Residual toner, remaining on the photosensitive drum 101 after the transfer, is removed from the photosensitive drum 101 by the cleaner 107.
Also the toner not used in the development at the contact portion between the photosensitive drum 101 and the developing roller 110 and remaining thereon is returned into the developing device 104 by the rotation of the developing roller 110. Then, at the contact portion between the developing roller 110 and the removing-feeding roller 111, the toner on the developing roller 110 is removed from the developing roller 110. At the same time, by the rotation of the removing roller 111, the toner is supplied onto the developing roller 110 and is conveyed again to a contact portion between the regulating blade 112 and the developing roller 110.
An image formation is achieved by the repetition of the above-described operations.
In recent years, energy saving and environmental issues are actively considered also in the image forming apparatus, and, as a part of such activities, there are being conceived toner fixable at a lower temperature than in the conventional toner, and toner incorporating fixing wax, for the purpose of dispensing with the fixing wax. However such toner, being designed to fuse at a lower temperature, is associated with a drawback of susceptible to friction and heat.
When such toner, fusing at a low temperature, was used in an image forming operation in a conventional developing apparatus, the toner was fused and fixed to the developing roller, the removing-feeding roller and the regulating blade whereby the toner could not be coated uniformly on the developing roller and a desired image could not be obtained.
Also in case of toner fusion to the developing roller or the regulating blade, there results a mutual frictional charging of the toner particles, whereby an appropriate charge cannot be provided to the toner. As a result, uncharged or deficiently charged toner is conveyed onto the developing roller and is deposited in a non-image area on the photosensitive drum, thereby causing an image defect, or is scattered onto the photosensitive drum or in the image forming apparatus, thereby causing a smear.
As a result of investigation by the present inventors for the cause of the above-mentioned drawback, there is identified, as a principal cause, a phenomenon that the toner removed from the developing roller by the removing-feeding roller is not discharged from the cells at the surface thereof but is directly conveyed again by the rotation of the removing-feeding roller to the contact portion with the developing roller, thereby causing a rapid deterioration of the toner.
Such deterioration of the toner principally means a deformation of the toner or a seeping out of the internal wax of the toner by such deformation. This is caused by accumulation of a frictional heat, generated by friction on the toner under a pressure in contact portions of various members, in the toner, thus leading to a softening of the toner. Thus, such toner softening occurs when the toner is subjected to frictions in succession in contact portions of the removing-feeding roller and the developing roller or of the developing roller and the regulating blade thereby receiving frictional heat in such contact portions in successive manner.
In case such toner softening and deformation by the frictional head proceed repeatedly, the toner is fused and sticks to the developing roller and the regulating blade, thereby resulting in aforementioned drawbacks that the toner cannot be uniformly coated on the developing roller or that the charge providing ability of the developing roller and the regulating blade is lowered.
Also, a charge control agent, added to the surface of the toner particles, may be embedded in the interior thereof or peeled off therefrom by such toner softening, whereby the charging ability of the toner itself is lowered and uncharged toner particles are easily generated. As a result, an image defect or a toner scattering is induced.
The above-mentioned drawbacks become conspicuous in the toner having a low fusing temperature and susceptible to heat.
An object of the present invention is to provide a developing apparatus capable of preventing generation of fused or uncharged developer by a deterioration of the developer.
Another object of the present invention is to provide a developing apparatus capable of stable development over a prolonged period.
Still another object of the present invention is to provide a developing apparatus capable of removing a developer, removed by a developer removing-feeding member, from the developer removing-feeding member.
Still another object of the present invention is to provide a developing apparatus capable of maintaining an appropriate charge on the developer.
Still another object of the present invention is to provide a developing apparatus, in which, even with a developer of a low fusing temperature, the developer is not softened on a developer bearing member or on a developer removing-feeding member.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings.